1. Field of the Invention
The present invention relates to a coaxial type capacitor suitable for use in a pulse generator which is used for measuring electrical and/or mechanical properties of various electric and electronic devices.
2. Description of the Prior Art
Conventionally, a pulse generator having a structure as shown in FIG. 13 is well known. Namely, in the pulse generator, a discharge gap G to which a capacitor Co and a trigger terminal 1 are connected, a resistor R.sub.S for restraining a high-frequency oscillation generated upon discharge and a discharging resistor R.sub.O are connected so as to form a closed loop, and a capacitor C.sub.f is connected in parallel to the resistor R.sub.O which composes an output circuit together with the capacitor C.sub.f. A pulse is generated across the resistor R.sub.O to which a coaxial connecting plug 3 of a coaxial cable 2 is connected for transmitting the generated pulse. The capacitor C.sub.O is charged through a charging resistor Rc by a direct current supplied from a charging terminal 4. Charge accumulated in the capacitor Co is discharged instantly through the discharge gap G, resistors R.sub.S and Ro and the capacitor C.sub.f at the moment that a trigger voltage is applied to the trigger terminal 1 of the discharge gap G and a pulse thus generated is outputted through the coaxial cable 2.
In order to obtain a fast rising property of the pulse, it is desirable to design the pulse generator so as to minimize the dimensions thereof and the total length of wiring. However, in the conventional pulse generator, the discharge gap G is formed with an air gap or a vacuum tube having relatively large dimensions and, further, bulky capacitors of normal type, each of which is housed in a metal case, are used as capacitors Co and Cf, as shown in FIGS. 14(a) and 14(b), and are connected independently according to the wiring diagram shown in FIG. 13. Accordingly, the length of wiring becomes long and this causes the inductance thereof to increase inevitably. As shown in FIG. 15 which shows an equivalent circuit of the pulse generator at the time of discharge of the capacitor Co, a residual inductance L of about 1.0 .mu.H.about.3.0 .mu.H is generated on the way of discharge by the wiring. The residual inductance can be reduced by improving structures of the pulse generator, but it cannot be reduced lower than 0.6 .mu.H.about.0.7 .mu.H. Due to this residual inductance, a high-frequency oscillation is superposed upon a pulse to be outputted and, therefore, the quality of the pulse becomes inferior.
As mentioned above, the resistor R.sub.S is inserted between the discharge gap G and the capacitor C.sub.f for restraining a high-frequency oscillation in the conventional pulse generator assuming that a residual inductance of 1.0 .mu.H.about.3.0 .mu.h is inevitable. However, the resistor R.sub.S must have a high resistance in order to restrain the high-frequency oscillation effectively. Due to this, the rising property waveform of a pulse which is generated by the pulse generator becomes dull and the output impedance of the pulse generator high. For reference, the rising time of a pulse is about .mu.S at the minimum according to the conventional pulse generator.